This research participates in the ongoing debate about the degree of engagement of public librarians in challenges to dominant social and cultural constructs. This research demonstrates that public librarians involved with the left-led CIO during the late 1930s and through the 1940s collaborated to create an alternative historical bloc that challenged the cultural hegemony dominant within the library profession, within local communities, and encompassing broader societies as a whole. This left-led affiliation was part of a broader cultural front, which emerged from the radical left engagement with the liberal center during the period known as the Popular Front. Within public libraries this front promoted strategies of collective action to engage social issues. The result was not only the professional commitment to intellectual freedom, codified within library policies, but a stronger identification with the diverse service populations of the American public library.